The present invention relates to a drive device for stepper motors applicable to an on-board combination meter and so on and to an indicating apparatus using the drive device.
For example, JP-A-2001-327193 discloses a drive device for controlling the driving of a plurality of stepper motors for use in a tachometer, a fuel meter and so on in an on-board combination meter of the sort mentioned above. The drive device includes a plurality of stepping motors, a plurality of driven members, a plurality of stoppers, a plurality of first excitation members, a plurality of second excitation members, a plurality of detection coils, a position detection member, and control member. Each of the stepping motors has a plurality of excitation coils as well as a rotor magnetized to have N and S magnetic poles and is rotated by following variations in the excitation state of the excitation coils. The driven members moves in response to the rotating operation of the respective rotors. The stoppers mechanically stop the respective driven members in position. The first excitation members rotate the rotors forward and backward by controlling the plurality of excitation coils. The second excitation members control the excitation state of the plurality of excitation coils according to exciting patterns constituted of a plurality of exciting steps for defining the excitation state of the plurality of excitation coils and which reversely rotate the rotors in the direction in which the driven members move to the predetermined positions. The detection coils generate induction voltage in response to the rotation of the rotors. The position detection member sequentially detect the presence or absence of the induction voltage generated in the detection coils during the control operation performed by the second excitation members and detect whether the driven members stop at the predetermined positions where the contact of the driven members against the stoppers is established on the basis of the presence or absence of the detected induction voltage. The control member stops the control operation performed by the first excitation members, and causes the second excitation members to start controlling when the control member receives a command signal. The control member stops the control operation performed by the second excitation members and causes the plurality of first excitation members to start controlling when all the driven members stop at the predetermined positions by the position detection member.
In the drive device described in for example, JP-A-2001-327193, the plurality of stepper motors are such that the reset operation of restoring the driven members (pointers) in position (the zero positions) by a command signal based on ignition is started by a sequential predetermined timing deviation (e.g., a deviation of two excitation steps) and then each stepper motor is shifted to the normal operation wherein the stepper motor is rotated forward or backward according to measured values of the vehicle speed, the number of revolutions and so on after the set operation of the whole stepper motor is terminated.
On the other hand, a drive device of another type is such that the sequence of zero-position detection of a plurality of stepper motors used in an on-board combination meter is controlled so that starting the detection of the zero positions is simultaneously made in the whole built-in stepper motor. In the drive device of this type, in order to deal with the time difference required to detect the zero positions of the stepper motors, it is arranged that any stepper motor that has terminated the zero-position detection waits for the termination of the zero-position detection by any other stepper motor and that each of the stepper motors simultaneously starts giving an indication in unison at a point of time the whole stepper motor terminates the zero-position detection.
In the above-described drive device, the source voltage supplied to a computer within the drive device, used to control the drive device drops because of cranking when a starter is turned to subject an engine to the cranking in order to start the engine in such a state that a fuel meter together with a tachometer and a speedometer contained in the on-board combination meter has started giving an indication according to the measured value. When the computer is reset for preventing malfunction due to the source voltage drop, the computer thus reset causes the stepper motors to start zero-position detection process operation at the time the, power is supplied again immediately after the resetting. With the pointer deflected to an indicating value immediately before the resetting in the fuel meter, it takes time to detect the zero position until the pointer returns to the zero position. Therefore, the tachometer that should start giving an indication corresponding to the measured value after the engine is started cannot start normal operation for giving any indication corresponding to the measured value when the power is supplied to the drive device again until the zero-position detection process operation is terminated in other stepper motors that take time to perform the operation.
FIG. 6 is a time chart illustrating the driving of a plurality of stepper motors by a conventional drive device by way of example. With a stepper motor 111 acting as a tachometer, a stepper motor 112 acting as a fuel meter and a stepper motor 113 acting as a speedometer, a drive device starts performing a zero-position detection process at time t1 but does not start performing normal operation for giving an indication corresponding to the values measured by the stepper motors 111 and 113 even though the stepper motors 111 and 113 terminate the zero-position detection process at time t2. The drive device just waits for the delayed termination (time t4) of the zero-position detection process performed by the stepper motor 112 and then assumes control so that the stepper motors 111, 112 and 113 start the normal operation in unison at time t5.
As the tachometer operates not to give any indication of the number of revolutions of the engine immediately after the engine is started in the case described above, this nonconformity may give the driver a sense of discomfort.